derivation of Hesse's law
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Derivation of Hess's Law
Hess's law is a relationship in physical chemistry named after Germain Hess, a Swiss-born Russian chemist and physician. ... By Hess's law, the net change in enthalpy of the overall reaction is equal to the sum of the changes in enthalpy for each intermediate transformation: ΔH = ΔH1+ΔH2+ΔH3.
What is Hess Law?
Hess’ law also known as Hess’s law of constant heat summation states, “at constant temperature, heat energy changes (enthalpy – ΔHrec) accompanying a chemical reaction will remain constant, irrespective of the way the reactants react to form product”.
Hess’ law is based on the state function character of enthalpy and the first law of thermodynamics. Energy (enthalpy) of a system (molecule) is a state function. So, enthalpy of reactant and product molecules is a constant and does not change with origin and path of formation.
The first law of thermodynamics states that the total energy of the substances before and after any (physical or chemical) change should be equal. According to the law, the total energy of the reactant should be equal to the total energy of the product. Any difference in the energy between the reactants and products is also fixed at a particular temperature and will not change with the path followed by the reactants to form products. Hence, heat energy also can be considered as a reactant or product of the reaction and included in the reaction.
Hence, exothermic reactions can be written as: A + B → C + D + ΔH
Similarly, endothermic reactions become: A + B + ΔH → C + D
This allows reactions containing reactants and products to be treated as algebraic equations and carrying out mathematical operations on them. It should be remembered, that an exothermic reaction in one direction will be endothermic in the reverse direction and vice-verse.
Importance of Hess Law
Every substance (atom/ molecules) possess energy within. The internal energy depends on the nature of force existing in the substance and the temperature. When the substance undergoes chemical reactions, some bonds connecting some atoms are broken and some bonds are made new. The breaking and making of bonds involve energy.
So, in reactions, product substances may have either less or the same or more energy than the reacting substances. Reactions accordingly may release heat to become exothermic or absorb heat and endothermic. Reactants may further react to give the product;
In single-step or
In multi-steps or
Along with other products.
Knowledge of the energy changes in any reaction is essential for the manipulation of the reactants and products in a chemical process to our requirement.
Heat energy changes of reactions measured at constant volume are called internal energy change ΔE and energy measured at constant pressure is called enthalpy change ΔH.
The experimental measurements give only the net value of all reactions or products formed. It is not possible to measure experimentally the enthalpy change of an intermediary reaction step or any intermediary product.
For example, carbon reacts with oxygen to form carbon dioxide in excess oxygen. Carbon and oxygen combine to form carbon dioxide directly or in two steps -initially form carbon monoxide and then go to carbon dioxide. Measurement will give the energy changes for the formation of carbon dioxide only and not for carbon monoxide.
Similarly, measuring the enthalpy of formation of benzene, from carbon and hydrogen is not possible, because carbon and hydrogen may combine to form not only benzene but also other types of hydrocarbons in the given conditions.
Hess’s law is useful and is the only way of calculating such non-measurable enthalpy changes in physical and chemical changes
Hope it helps u☺☺